Suicide grip.
One day the guy will lift heavy and get tired and hape slippery palms and the bars gonna drop on his neck and decapitate him.
Then he can go sue the PT for giving him that advice.
I use the thumless grip when I know I'm within my limits, feels better.
But, would never use it when pushing my limits.
So to say "never curl your thumbs around the bar" is just bad advice.
There are plenty of Youtube clips of the bar crushing peoples necks and chest from accidents using this grip because they tried to lift heavy.